While from a legal standpoint you are fine in Russia, I think they are worried about what it means in EU and if they invest any money into pixels that it might be taken away at a later time, or that it might be legal issues. They are asking these points for you, not for them selves.
Nothing about how to get pixels, or the randomness of getting items makes it gambling. It's the fact that it's tradable for other currency(steam$$) makes it gambling. Take away the trading and you take away the gambling factor.
This is how it's gambling:
Money->Pixels->Art->Steam Inv -> Money -> Pixels.
Take away the Steam inv and replace it with in-game trading and it might take away the money part of this equation:
The current graffiti is just random graphics, it would be cool if it was more screeps related, and maybe even have contests for players to draw vector art that can be voted on and put in the game.
Why is there no graffiti with actual game art? You have all these cool designs and stuff in the GUI, is this not available?